Add Go test code coverage

.github/workflows/coverage.yml

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+name: coverage
+
+on:
+  push:
+    branches: [main]
+  pull_request:
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.head_ref || github.run_id }}
+  cancel-in-progress: true
+
+env:
+  GO_VERSION: 1.24.x
+
+jobs:
+  coverage:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+
+      - name: Set up Go
+        uses: actions/setup-go@v4
+        with:
+          go-version: ${{ env.GO_VERSION }}
+
+      - name: Go cache
+        uses: actions/cache@v4
+        with:
+          path: |
+            ~/go/pkg/mod
+            ~/.cache/go-build
+          key: ${{ runner.os }}-go-cache-${{ hashFiles('**/go.sum') }}
+
+      - name: Compute coverage
+        run: make coverage
+
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v4
+        with:
+          files: go/coverage.out
+          flags: unittests
+          fail_ci_if_error: true
+          verbose: true
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+

go/.gitignore

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+coverage.*

go/go.work

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+go 1.24.9
+
+use (
+	../benchmarks
+	../examples/ints
+	../examples/jsonl
+	../examples/profile
+	./grpc
+	./otel
+	./pdata
+	./pkg
+	../otelcol
+	../stefc
+	../stefc/generator/testdata
+)